Synthetic Fibres and Plastics
Synthetic Fibres and Plastics
The final chemistry topic gives 1 question, usually on identifying a fibre or on the environmental problem of plastics. It is a short topic with a fixed set of names, so it is among the cheapest marks in the science block.
Natural and Synthetic Fibres
Natural fibres come from plants or animals — cotton, jute, wool and silk. Synthetic fibres are made in a factory from chemicals derived largely from petroleum, and are also called man-made fibres.
| Fibre | Made from or known as | Common uses |
|---|---|---|
| Rayon | Wood pulp; artificial silk | Textiles, mixed with cotton |
| Nylon | Fully synthetic; first such fibre | Ropes, parachutes, socks, seat belts |
| Polyester | Fabric that resists creasing | Terylene, PET bottles |
| Acrylic | Artificial wool | Sweaters, blankets |
Rayon is the odd one out and is asked because of it: although manufactured, it is made from a natural source, wood pulp, so it is called a semi-synthetic or regenerated fibre. Nylon was the first fully synthetic fibre and is stronger than a steel wire of the same thickness, which is the fact examiners like.
✗ Rayon is a fully synthetic fibre made from petroleum | ✓ Rayon is made from wood pulp, so it is semi-synthetic
Why Synthetic Fibres Are Used
- They dry quickly, are durable and relatively inexpensive.
- They resist creasing and are easy to maintain.
- They are less absorbent, which makes them uncomfortable in hot weather.
- They melt on heating, which is why synthetic clothes should not be worn while working in a kitchen or laboratory.
That last point is a safety rule a science teacher must know, and it is asked directly rather than as a property.
Plastics
Plastic is a polymer that can be moulded into shape. The important classification is by behaviour on heating.
| Type | Behaviour | Examples | Uses |
|---|---|---|---|
| Thermoplastic | Softens on heating, can be remoulded | Polythene, PVC | Bottles, toys, combs |
| Thermosetting | Sets permanently, cannot be remoulded | Bakelite, melamine | Electrical switches, cookware handles, floor tiles |
Bakelite is used in electrical switches because it is a poor conductor of heat and electricity, and melamine resists fire, which is why it is used for kitchenware and fire-resistant uniforms. Both pairings are asked with the reason attached.
Plastics and the Environment
Plastic is non-biodegradable, meaning micro-organisms cannot break it down. This is its greatest advantage as a material and its greatest problem as waste.
- Burning plastic releases poisonous gases, so it should never be burnt in the open.
- Discarded plastic blocks drains and is swallowed by stray animals, often fatally.
- The remedy is summarised as the four Rs: reduce, reuse, recycle, recover.
- Plastic bags below a specified thickness are banned in many states for this reason.
The four Rs formulation is asked as a direct question, and it is also the natural teaching angle — this is a topic a class 8 teacher is expected to turn into a classroom discussion rather than a list of names.
रेयॉन लकड़ी के गूदे से बनता है, इसलिए पूर्णतः संश्लेषित नहीं है।
TRE pointer: Two items decide this topic: rayon comes from wood pulp and is therefore not fully synthetic, and the thermoplastic versus thermosetting distinction with bakelite and melamine as the examples. The four Rs is a direct one-line question. Because plastics are an environment topic, expect it to be paired with a conservation angle rather than pure chemistry. With five options and −1/3 negative marking, these fixed names are worth locking down completely.
60-Second Recap
- Natural fibres come from plants and animals; synthetic fibres are made from chemicals.
- Rayon is made from wood pulp, so it is semi-synthetic; nylon was the first fully synthetic fibre.
- Synthetic fibres melt on heating, so they are unsafe near a flame.
- Thermoplastics such as polythene and PVC can be remoulded; thermosetting plastics cannot.
- Bakelite is used in switches for poor conductivity; melamine resists fire.
- Plastic is non-biodegradable; the remedy is reduce, reuse, recycle and recover.
Frequently Asked Questions
Is rayon a natural or a synthetic fibre?
Neither entirely. Rayon is manufactured in a factory but from wood pulp, which is a natural source, so it is classed as semi-synthetic or regenerated. It is also called artificial silk because of its appearance and feel, and this classification is asked directly.
What is the difference between thermoplastic and thermosetting plastic?
A thermoplastic such as polythene or PVC softens when heated and can be remoulded repeatedly. A thermosetting plastic such as bakelite or melamine sets permanently on first moulding and cannot be softened again, which is why it is used for switches and cookware handles.
Why should synthetic clothes not be worn while cooking?
Because synthetic fibres melt rather than burn away, and molten fibre sticks to the skin and causes severe burns. Cotton, being a natural fibre, does not behave this way. This is a safety rule that science teacher examinations ask about directly.
What are the four Rs of plastic waste management?
Reduce, reuse, recycle and recover. Plastic is non-biodegradable, so micro-organisms cannot break it down and it persists for a very long time. Burning it releases poisonous gases, which is why disposal has to be managed rather than simply got rid of.
Next step: Chemistry in Part-III is decided by tables and exceptions, and both fade fast without testing. Practise this chapter in the BPSC TRE 4.0 mock test on ExamAtlas — five options, −1/3 negative marking, Part-III depth.